ConciseErrors

ConciseErrors swaps Rails’ ActionDispatch::DebugExceptions middleware with a compact error page that focuses on the class, message, and a truncated backtrace. It was designed to give human developers and AI assistants the minimum viable context needed to diagnose a failure without the noise of the default HTML-heavy debugger.

• Tiny plain-text payloads by default (no inline assets, no copious markup).
• Optional single-file HTML view optimised for dark/light mode without external CSS.
• Configurable backtrace depth with an omission indicator.
• Automatic fallback to text/plain when the request is xhr? or clients negotiate non-HTML Accept headers.
• HTML view includes a button to open Rails' full-featured error page.

Add the gem to your Gemfile:

gem "concise_errors"

Run bundle install and restart your Rails server. ConciseErrors automatically swaps the middleware when the gem loads.

ConciseErrors ships with opinionated defaults — HTML output, no CSS, and Web Console middleware is automatically removed in development — so simply installing the gem is enough. Override anything you need from config/application.rb or an environment-specific config:

# config/environments/development.rb
Rails.application.configure do
  config.concise_errors.tap do |cfg|
    cfg.stack_trace_lines = 5   # default: 10
    cfg.format = :html          # available: :text (default) or :html
    cfg.enabled = true          # flip to false to restore the stock debug page
    cfg.application_root = Rails.root.to_s # optional: trim this prefix from traces
    cfg.logger = Rails.logger              # optional: reuse your preferred logger
    cfg.full_error_param = "concise_errors_full" # optional: query param to trigger full page; set nil/"" to disable
  end
end

You can also steer the default format via ENV["CONCISE_ERRORS_FORMAT"] (text or html).

ConciseErrors only affects the debug middleware (the screen you see when config.consider_all_requests_local is true). Production 500 pages continue to use whatever ActionDispatch::ShowExceptions is configured to serve.

When rendering HTML, ConciseErrors shows a "View full Rails error page" button. Clicking it performs a same-origin request back to the same URL with the concise_errors_full=1 flag (or your configured full_error_param). The original request method, CSRF token, and content type are preserved; for non-GET/HEAD requests the request body is replayed.

• To disable the button entirely, set config.concise_errors.full_error_param = nil or "".
• If Web Console is installed, the fallback will go through its middleware; otherwise Rails' stock ActionDispatch::DebugExceptions page is shown.

Plain text format (default):

RuntimeError: bang
HTTP 500 (Internal Server Error)

app/controllers/widgets_controller.rb:12:in `show'
app/controllers/widgets_controller.rb:12:in `show'
...

HTML format renders the same content inside a single <pre> block with minimal inline styling suitable for AI agents that prefer HTML responses.

After checking out the repo run:

bin/setup
bundle exec rake test

bin/console opens an IRB session with the gem loaded. To try the middleware in a real app, add a path: entry to a Rails application's Gemfile pointing at your local clone.

Before cutting a release:

1. Update ConciseErrors::VERSION and CHANGELOG.md.
2. Run bundle exec rake release to tag, build, and push the gem.

Bug reports and pull requests are welcome on GitHub at https://github.com/obie/concise_errors. All contributions are expected to follow the code of conduct.